The present invention relates to a double screw rotor assembly, and more particularly to a multi-segment or combination double screw rotor assembly for controlling a flow pressure, for example, for use in vacuum pumps, air compressors, etc.
FIG. 1 shows a double screw rotor assembly constructed according to U.S. Pat. No. 5,443,644. This structure of double screw rotor comprises two screw rotors 81 and 82 meshed together. Because the screw rotors 81 and 82 have an uniform pitch Pxe2x80x2 and same height of tooth Hxe2x80x2, the volume and pressure of the air chambers 810 and 820 are not variable. When operated through a certain length of time, a high pressure occurs in the area around the outlet 80, and a significant pressure difference occurs when air is transferred to the outlet 80, resulting in a reverse flow of air, high noises, and high energy consuming.
U.S. Pat. No. 5,667,370 (FIG. 2) discloses a horizontal type double screw rotor assembly. According to this design, the first pair of screw rotors 32xe2x80x2 and 33 and the second pair of screw rotors 34 and 35 have different outer diameters and pitches. Further, the installation of the partition plate 93 between two shells 91 and 92 greatly increases the dimension of the screw rotor assembly and complicates its structure.
FIG. 3 shows still another structure of horizontal type double screw rotor assembly according to the prior art. According to this design, the screw rotors 4xe2x80x2 and 5xe2x80x2 have a variable pitch. However, because the processing of the screw rotors requires a specially designed processing equipment and cutting tool, the manufacturing cost of this structure of double screw rotor is high.
The present invention has been accomplished to provide a combination double screw rotor assembly, which eliminates the aforesaid drawbacks. It is one object of the present invention to provide a combination double screw rotor assembly, which effectively prevents a reverse flow, and reduces power loss and operation noise. It is another object of the present invention to provide a combination double screw rotor, which is compact and requires less installation space. It is still another object of the present invention to provide a combination double screw rotor assembly, which is easy and inexpensive to manufacture. According to one aspect of the present invention, the combination double screw rotor assembly comprises a casing, a first screw rotor, and a second screw rotor. The casing comprises an inside wall defining a receiving chamber, an inlet, and an outlet. The first rotor comprises a shaft pivoted in the casing, a low pressure screw rotor element and a high pressure screw rotor element respectively mounted on the shaft in a direction from the inlet toward the outlet, and a spiral thread raised around the periphery thereof and extended over the low pressure screw rotor element and high pressure screw rotor element. The spiral thread of the first rotor is comprised of a first spiral thread segment raised around the periphery of the low pressure screw rotor element of the first rotor and defining a uniform long pitch, and a second spiral thread segment raised around the periphery of the high pressure screw rotor element of the first rotor and defining an uniform short pitch. The second screw rotor comprises a shaft pivoted in the casing and disposed in parallel to the shaft of the first screw rotor, a low pressure screw rotor element and a high pressure screw rotor element respectively mounted on the shaft of the second rotor in a direction from the inlet toward the outlet, and a spiral thread raised around the periphery thereof and extended over the low pressure screw rotor element and high pressure screw rotor element of the second rotor. The spiral thread of the second rotor is comprised of a first spiral thread segment raised around the periphery of the low pressure screw rotor element of the second rotor and defining a uniform long pitch, and a second spiral thread segment raised around the periphery of the high pressure screw rotor element of the second rotor and defining a uniform short pitch. The first spiral thread segment and second spiral thread segment of the spiral thread of the second screw rotor are respectively meshed with the first spiral thread segment and second spiral thread segment of the first screw rotor. According to another aspect of the present invention, two parallel sets of axle bearings are mounted in the casing near the outlet to support the shafts of the first screw rotor and the second screw rotor, and keyless axle bushes or like device are installed in the shafts of the first screw rotor and the second screw rotor to secure the axle gearings in place. According to still another aspect of the present invention, timing gears are respectively mounted on the shafts of the first screw rotor and the second screw rotor and meshed together for enabling the first screw rotor and the second screw rotor to be rotated without contact.